Extruded polyolefin pipes are well known for a variety of industrial applications. Typically they are used in the building industry for domestic water pipes, radiator pipes, floor-heating pipes and for similar applications in ship building etc. Polyolefin pipes can also be used as district heating pipes and as process pipes in the food industry etc. Other applications include the conveyance of gaseous fluids and slurries.
Multilayer pipes wherein at least one of the layers comprises an extruded polyolefin are also well known and a great many have been described in the literature. Multilayer pipes are used, for example, when improved long term strength at elevated temperatures is needed or, when barrier properties against oxygen permeation are necessary. Multilayer pipes can comprise dissimilar materials for particular applications. For example, multilayer pipes having diffusion barrier layers have been proposed. The diffusion barrier can be a polymeric layer such as EVOH, or a metallic layer which provides both a diffusion barrier and a strengthening layer.
In recent years multilayer pipes having aluminium based barrier layers have become very popular. When installing domestic heating systems the metal barrier provides a specific and important benefit, which is that when the pipe is bent it retains its new configuration, in contrast to plastics pipes without a metal barrier layer, which tend to recover their original shape.
However, multilayer plastics pipes comprising two or more layers of polyolefin homopolymers or copolymers having an intermediate metallic barrier or strengthening layer disposed between them tend to have poorer performance over the long-term than, for example, PEX pipes, comprising a single layer of cross-linked polyethylene. In addition, the difference between the coefficients of thermal expansion of a metallic barrier layer and the plastics layers can lead to delamination. Nevertheless, the presence of a metal barrier layer is often very desirable in certain applications of plastics pipes, for example, in domestic and district heating and in the oil, petroleum and gas industries. Multilayer plastics pipes with metal barrier layers also find use in cold water applications where potable water needs to be protected from aromatic substances found in the soil.
A further benefit of plastics pipes with metallic barrier layers is that the metal layer prevents UV light from reaching the inner plastics layer(s) beneath it, thereby protecting these layer(s) from UV degradation. This protection obviates the need for the addition of UV stabilisers to the inner layer(s) and enables the stabiliser packages of the inner and outer plastics layers to be optimised, with the inner layer(s) requiring only thermal and chemical stabilisation. Examples of plastics pipes having metal barrier and strengthening layers and methods for their manufacture are disclosed in the following patents:
CH655986JP93-293870EP0644031EP0353977EP0581208
The entire disclosures of which are incorporated herein by reference for all purposes.
Typical multilayer pipe constructions consist of five layers where the innermost layer comprises, for example, PE-RT (polyethylene for higher temperatures), which is overlaid with a first adhesive layer, an overlapped or butt welded aluminium strengthening and barrier layer, a second adhesive layer and an outer layer of PE-RT or silane cross-linked PEX (cross-linked polyethylene). The adhesive layers are necessary because many polymers, including polyolefins, have very poor adhesion to aluminium.
This construction has several drawbacks. Firstly the inner plastics layer and the first adhesive layer are together rather thin and in some manufacturing processes the thickness of the first adhesive layer is difficult to control.
Secondly the first adhesive layer is usually made of a thermoplastic polymer that is mechanically weaker than the inner plastics layer and hence does not improve the long-term hydrostatic strength of the pipe. This means in practice that omitting the first adhesive layer would provide advantages in the form of improved long term strength, easier quality control and easier extrusion tool design.
Thirdly, in manufacturing processes wherein the inner plastics layer is directly extruded into a freshly formed and welded aluminium tube comprising the barrier layer, the thermal shrinkage of the hot extruded inner plastics layer tends to cause delamination, requiring the use of a high strength adhesive as the first adhesive layer.
It has been proposed to limit the thermal shrinkage of a thermoplastic polymer by compounding relatively large particle size fillers into the polymeric matrix. However, the loading level needs to be rather high in order to reach the desired effect and this reduces the flexibility of the pipe. The use of high levels of filler also introduces further problems, including the difficulty of obtaining good wetting of the filler by the polymeric matrix, which is necessary in order to obtain good mechanical properties. Polyolefins, for example, are non-polar and incompatible with hydrophilic fillers. Thus, poor adhesion between the filler surface and the matrix is a frequent outcome.
Some improvement in the wetting of the filler surface by the polymeric matrix may be obtained by the use of coating agents, for example, fatty acids such as stearic acid, and salts of fatty acids, which can react with, for example, hydroxyl groups on the filler surface, but further improvements would be highly desirable.
Stabilisation of thermoplastic polymers is usually accomplished by melt blending with one or more stabilisers. In this way a heterophase polymer/stabiliser system is formed, which may be best described as a physical dispersion of a low molecular weight stabiliser in a polymer matrix. The vast majority of commercial stabiliser compounds have very different chemical structure from that of the non-polar host thermoplastic polymer. For this reason, the compatibility of various conventional stabilisers with polyolefins is poor, leading to migration of admixed stabilisers across the boundary of the polyolefin with neighbouring fluids, liquids, gases or solid materials. This loss of stabiliser substantially shortens the lifetime of the polyolefin. The migration of stabilisers into drinking water can also have unpredictable toxic effects on consumers.
The long-term performance of plastics pipes is typically evaluated using the SEM method where the pipe is pressurised at elevated temperatures and the time to burst is measured at different stress levels. Considerable research effort has been focused on so-called stage III ruptures, which take place when the stabiliser package has ceased to be effective. If the stabilisers can migrate and leach out of the matrix easily the long-term endurance of the pipe is jeopardised.
A method for studying stabiliser migration involves immersing the pipe in boiling water with subsequent measurement of the oxidation induction time (OIT) level, which gives an indication of how much active stabiliser is remaining in the pipe and measures how easily the stabiliser is able to leach out of the pipe wall. By measuring the OIT levels at different time intervals it is possible to estimate by extrapolation the lifetime of the pipe.
In US2001/0031324 there is described a plastic pipe comprising:
a tubular body comprised of an outer layer, an intermediate layer connected radially inwardly to the outer layer, and an inner layer connected radially inwardly to the intermediate layer, wherein the inner layer is in contact with a medium to be transported;
wherein the inner and the outer layer are comprised of a basic material and the intermediate layer is comprised of a composite material of a basic material and an additional material, wherein the basic material of the inner layer and of the intermediate layer is a polymer material, wherein the polymer material comprises amorphous areas;
additives against aggressive media embedded in the amorphous area of the polymer material of at least one of the inner layer and the intermediate layer;
wherein the additional material is a barrier material, selected from the group consisting of fillers and additives, embedded in the amorphous area of the polymer material of the intermediate layer and configured to reduce migration of the additives embedded in the amorphous area of the polymer material of the inner layer.
The barrier material proposed in US2001/0031324 is selected from glass fibres, glass beads, glass powder or mixtures thereof. It will be apparent that the presence of the barrier material in the intermediate layer does not prevent the additives against aggressive media present in the inner layer from leaching out into the transported medium.
It is apparent that there are several problems associated with existing multilayer plastics pipe constructions. In particular, improvements in adhesion of the inner polymer layer to any barrier or strengthening layer present and in reducing stabiliser leaching and migration would be highly desirable.